Makefile.in: By default repeat the task for all enabled MACHINEs

:Release Notes:
When user calls make clean-zlib, do it for all MACHINEs enabled by mcf.

:Detailed Notes:
Respect MACHINE, MACHINES, DISTRO variables when sourcing
oe-init-build-env script, this prevents overwritting the value set by
Makefile when calling bitbake.

:Testing Performed:
Only build tested.

:QA Notes:
No change to image.

:Issues Addressed:
[ES-1151] Simplify build-* Makefile

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I542905f05339bf59db5fee336bb69ad17a576265
Reviewed-on: https://g2g.palm.com/6379
Reviewed-by: DCO Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>